man: tc: Describe qevents

Add some general remarks about qevents.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David Ahern <dsahern@kernel.org>

man/man8/tc.8

@@ -254,6 +254,25 @@ Traffic control filter that matches every packet. See
 .BR tc-matchall (8)
 for details.
 
+.SH QEVENTS
+Qdiscs may invoke user-configured actions when certain interesting events
+take place in the qdisc. Each qevent can either be unused, or can have a
+block attached to it. To this block are then attached filters using the "tc
+block BLOCK_IDX" syntax. The block is executed when the qevent associated
+with the attachment point takes place. For example, packet could be
+dropped, or delayed, etc., depending on the qdisc and the qevent in
+question.
+
+For example:
+.PP
+.RS
+tc qdisc add dev eth0 root handle 1: red limit 500K avpkt 1K \\
+   qevent early_drop block 10
+.RE
+.RS
+tc filter add block 10 matchall action mirred egress mirror dev eth1
+.RE
+
 .SH CLASSLESS QDISCS
 The classless qdiscs are:
 .TP